Method for determination of resistivity of N-Type silicon Epitaxial layer

Abstract: A method for the determination of the resistivity of an n-type epitaxial layer formed on a silicon substrate is disclosed. This invention resides in either directly determining the true resistivity of a sample by preparing this sample without a natural oxide film which is responsible for the change with the passage of time or indirectly determining the true resistivity of a sample by intentionally forming on the sample a natural oxide film so stable to defy the change with the passage of time and measuring resistivity of this sample.

Description: Field of the Invention: This invention relates to a method for the determination of the resistivity of an n-type silicon wafer. More particularly, this invention relates to a method for the determination of the true resistivity of an n-type silicon epitaxial layer by the four point probe method. Description of the Prior Art: As a way of determining the resistivity of a silicon epitaxial layer grown on a silicon substrate, the four point probe method illustrated in Fig. 5 has found wide spread acceptance. This method determines the resistivity of a sample S by arranging four point probes 1 to 4 equidistantly on the surface of the sample S, forming a flow of a direct current I from the two outer probes to the sample S, and measuring the voltage V which occurs consequently between the two inner probes. In Fig. 10, N stands for an n-type region, P for a p-type region, t for the film thickness of an epitaxial layer, and 5 for a p-n junction. By this method of determination, the resistivity ρ of the epitaxial layer can be obtained in accordance with the following formula (1): ρ = aRt wherein a stands for the constant (π /ln2), R for the sheet resistance of the epitaxial layer, and t for the film thickness of the epitaxial layer. In the case of the n-type epitaxial layer, the sheet resistance R is determined by obtaining growth of this n-type epitaxial layer on a p-type silicon substrate and actually measuring the sheet resistance by the four point probe method. The film thickness t of the epitaxial layer is generally determined by measuring the film thickness of an epitaxial layer which is grown on a high-concentration antimony substrate in accordance with the Fourier-Transform Infrared Spectroscopy (FTIR) method. The resistivity ρ is the product of R multiplied by t and a, a constant. SUMMARY OF THE INVENTION Incidentally, the resistivity ρ (sheet resistance R) is prone to the so-called time-course change, a phenomenon that this magnitude increases with the elapse of time, depending on the specification of the n-type epitaxial layer to be manufactured. The magnitude thus rising requires more than one week's time before it is stabilized. The time-course change tends to increase in accordance as the film thickness of the epitaxial layer decreases and the resistivity increases. By way of specific illustration, the results of a determination of the resistivity in accordance with the conventional method will be described below. Wafers having epitaxial layers 10, 25, 37, and 64 µm in film thickness were severally produced by growing an n-type <111> epitaxial layer of a target resistivity of 30 Ω cm to a film thickness of 64µm by the conventional method on the mirror polished surface of a p-type silicon wafer and then subjecting the epitaxial layer to vapor-phase etching using hydrochloric acid. By the conventional four point probe method, the epitaxial layers of these wafers were tested for resistivity. The results of this test were as shown in Fig. 4. Since the epitaxial layers were produced in one and the same batch, they should have exhibited resistivity of one and the same magnitude. Actually, this was not the case. Specifically, it is clearly noted from the test results that when the epitaxial growth is attained by the conventional method, the initial resistivity increases and the ratio of change thereof also increases in accordance as the film thickness of the epitaxial layer decreases because the surface of the epitaxial layer produces a natural oxide film. On account of this change with the passage of time, it is doubtful whether the actually measured resistivity represents a true magnitude or not. Thus, it is not improbable that the produced epitaxial layer will be rejected for apparently inferior quality. This phenomenon of aging also entails the disadvantage that the apparent inferiority of quality raises the problem of guaranty and exerts an adverse effect on the setting of production conditions. Further, an effort to determine the true resistivity of an epitaxial layer which is inherently prone to the change with the passage of time entrains the disadvantage that this determination must be performed immediately after the epitaxial growth. The present invention, produced in view of the true state of the prior art described above, has for an object thereof the provision of a method which enables the determination of the true resistivity of an n-type silicon epitaxial layer to be easily and conveniently attained by the four point probe method by virtue of a prescribed pretreatment to be given to a sample under test. Now, the discussion which the present inventors have devoted to the change of the resistivity of an n-type silicon epitaxial layer with the passage of time will be described in detail below with reference to Fig. 6. The method for determining the resistivity of the epitaxial layer through the four point probe method, as implied by the aforementioned formula (1), consists in finding the product of the sheet resistance R of the epitaxial layer measured by the four point probe method multiplied by the film thickness t of the epitaxial layer obtained by the FTIR and the constant a. The fact that the four point probe method unveils the occurrence of a change with the passage of time actually represents the increase of Δ ρ in resistivity due to the increase of Δ R in sheet resistance measured by the four point probe method. To be specific, the actually measured resistivity, ρ + Δ ρ , can be obained in accordance with the following formula (2). ρ + Δ ρ = a (R + ΔR ) t The film thickness t of the epitaxial layer which appears in the formula (2) is believed to be constant because the silicon substrate sample has not been heated after the process of epitaxial growth and, therefore, has not undergone any change from the metallurgical point of view. Incidentally, the true resistivity ρ which is fixed by the physical amount of a dopant is constant because it does not undergo any change with the passage of time. It is, therefore, logical to conclude from the electrical point of view that the film thickness of the epitaxial layer is decreased by Δ t in accordance with the increment Δ R in the formula (2) so as to ensure the constancy of ρ . Thus, the resistivity ρ can be found in accordance with the following formula (3). ρ = a(R +Δ R )(t - Δ t) This decrement Δ t is logically considered to have been induced not metallurgically but electrically and is specifically believed to be ascribable to the formation on the surface of a silicon substrate (silicon wafer) of a depletion layer 6 which is impervious to electric current as illustrated in Fig. 6 (the decrement Δ t representing the expansion of the depletion layer). The change of the resistivity with the passage of time is not observed in the p-type silicon but is observed exclusively in the n-type silicon. From this fact, it is inferred that the depletion layer 6 is formed because the natural oxide formed on the surface of silicon develops a negative electric charge. In fact, there have been obtained test results which endorse this inference. From the formula (1), the formula (2), and the formula (3), the following formulas can be derived. Δ ρ = a(R + Δ R)Δ t = a Δ Rt Hence, Δ t = [ Δ R/(R + Δ R)]t Further, the amount of negative charge Ns (/cm 2 ) in the natural oxide film which induces the increment Δ t satisfies the following expression, depending on the condition of charge neutrality. Ns = Δ t · Np (Np: Dopant concentration in epitaxial layer) From the discussion described above, it is concluded that the resistivity of the n-type silicon is changed with the passage of time because the negative charge which exists in the natural oxide film formed on the surface of an epitaxial layer increases with the elapse of time and, in consequence thereof, a depletion layer is formed on the surface of the epitaxial layer and the film thickness of the electrically effective epitaxial layer is decreased by Δ t. The present invention is based on this discussion. The essence of this invention resides in directly determining the true resistivity of a sample by preparing this sample without a natural oxide film which is responsible for the change with the passage of time. The present invention provides a method of determining the resistivity of an n-type epitaxial layer formed on a silicon substrate, which is characterized by subjecting the surface of the n-type epitaxial layer to hydrogen termination and then measuring by means of the four point probe method the sheet resistance of the surface of the n-type epitaxial layer whilst still in the hydrogen terminated state. A first embodiment of this invention is characterized by the steps of growing the n-type epitaxial layer on a silicon substrate, displacing the inner gas of an epitaxial growth reactor with hydrogen gas and cooling the silicon substrate to a temperature below 400°C thereby effecting the hydrogen termination of the surface of the n-type epitaxial layer, subsequently ' displacing the hydrogen gas in the epitaxial growth reactor with an inert gas and simultaneously cooling the silicon substrate to normal room temperature, removing the cooled silicon substrate from the epitaxial growth reactor, and measuring sheet resistance of the n-type epitaxial layer by the four point probe method. A second embodiment of this invention is characterized in that the silicon substrate is contacted with a dilute aqueous hydrofluoric acid solution to remove the natural oxide film on the surface of the n-type epitaxial layer and, at the same time, effect the hydrogen termination, rinsed with deionized (DI) water, dried, and subjected to measurement of the sheet resistance by the four point probe method. A third embodiment of this invention is characterized in that the product of the sheet resistance measured by the four point probe method is multiplied by the film thickness of the n-type epitaxial layer determined by the FTIR method or the spreading resistance (SR) method and the constant (π /ln2) to obtain the resistivity. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be better understood and the objects and features thereof other than those set forth above will become apparent when consideration is given to the following detailed description thereof, which makes reference to the annexed drawings wherein : Fig. 1 is a graph showing the results of Example 1. Fig. 2 is a graph showing the results of the conventional method and Example 1. Fig. 3 is a graph showing theresults of Example 1. Fig. 4 is a graph showing the change in the measured magnitude of the resistivity of an n-type epitaxial layer with the passage of time in connection with the results of the conventional method. Fig. 5 is an explanatory diagram illustrating the procedure for determination of the sheet resistance by the four point probe method. Fig. 6 is an explanatory diagram illustrating a depletion layer formed on the surface of an n-type epitaxial layer. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The method of determining the resistivity according to the invention allows the true resistivity of an n-type epitaxial layer to be determined accurately because the surface of the n-type epitaxial layer is maintained in the state of hydrogen termination during the measurement of the sheet resistance. The method of determining the resistivity according to the first embodiment of this invention, owing to the treatment for hydrogen termination, imparts to the surface of the silicon substrate such a state of hydrogen termination that formation of a natural oxide film on the surface proceeds very gradually and the state of hydrogen termination itself lasts stably for a long time even when the silicon substrate is stored under normal temperature and humidity conditions. By this reason, the silicon substrate which has been removed from the epitaxial growth reactor does not always need to be stored under the condition of low humidity but must only be subjected without delay to the measurement of sheet resistance under the normal temperature and humidity conditions to ensure accurate determination of the true resistivity. The surface of the silicon substrate which has been treated for hydrogen termination by the method of determining the resistivity according to the second embodiment of this invention suffers the formation of a natural oxide film to proceed slightly faster than the surface treated likewise for hydrogen termination by the method according to the second aspect of this invention. The method of the second embodiment of this invention nevertheless permits the true resistivity to be accurately determined by maintaining the sample in an atmosphere of prescribed temperature and humidity thereby enabling the surface of the n-type epitaxial layer to be maintained in the state of hydrogen termination. Even in the absence of the storage under the specific condition mentioned above, the accurate determination of the true resistivity can be attained infallibly because the determination is carried out before the formation of a natural oxide film takes place. Now, the present invention will be described in detail below with reference to working examples and comparative experiments. The working examples of this invention will be explained in comparison with the comparative experiments below. Example 1: A plurality of silicon wafers severally having n-type <111> epitaxial layers of target resistivity of 30 Ω cm grown to a film thickness of 15µ m were used as samples. These samples were left standing for a fixed length of time in atmospheres having a substantially constant temperature (normal room temperature) and varying degrees of humidity (50%, 60%, and 95% of relative humidity) and then tested for sheet resistance by the conventional procedure using a four point probe measuring apparatus. The film thickness of the epitaxial layer was determined by the SR (Test Run 1). Separately, a plurality of silicon wafers severally having magnitudes of resistivity converted to a prescribed value (due to completion of the change with the passage of time) were prepared, immersed for one minute in a dilute aqueous solution containing hydrofluoric acid at a concentration of not more than 5% by weight (aqueous HF solution), rinsed with DI water, dried at normal room temperature, then left standing for a fixed length of time in an atmosphere having a substantially constant temperature (normal room temperature) and varying degrees of humidity (50%, 60%, and 95% of relative humidity), and tested for resistivity by the same procedure as described above to examine the condition of change in resistivity due to the length of time of standing. The resistivity was determined also by the C-V method (Shottky method ) (Test Run 2). Example 2: In an epitaxial growth reactor adapted to utilize a vapor-phase reaction, p-type silicon wafers were set in place, n-type <111> epitaxial layer having a target resistivity of 30 Ω cm were grown to a film thickness of 15 µ m at the temperature normally used for the epitaxial growth of this kind, and then the inner temperature of the reactor was cooled to about 200°C with the interior of the reactor continuously swept with hydrogen. Subsequently, the interior of the reactor was displaced with nitrogen, the reactor was cooled to normal room temperature, and the silicon wafers were removed from the reactor and immediately tested for sheet resistance. The silicon wafers were then left standing for a fixed length of time in an atmosphere having a substantially fixed temperature (normal room temperature) and different degrees of humidity (50%, 60%, and 95% of relative humidity) and then tested for sheet resistance. Now, the results of the working examples cited above will be explained below. Example 1: The results of Test Run 1 indicate the trend that the resistivity and the change thereof with the passage of time both increase in proportion as the relative humidity of the atmosphere in which the sample is stored (not shown). The results of Test Run 2 deny the presence of a discernible change of resistivity with the passage of time in an atmosphere having a relative humidity of 50% and indicate that they coincide substantially those determined by the C-V method as shown in Fig. 1. In consideration of the fact that this C-V method unveils no discernible change of the resistivity of an n-type epitaxial layer with the passage of time, it is safeiy concluded that the method of Test Run 2 is capable of determining the true resistivity of the epitaxial layer. The change of resistivity with the passage of time in an atmosphere of a relatively high humidity such as, for example, 60% is as represented by the curve la in Fig. 2. It approximates the change of the curve 1b which occurs when the sample after the epitaxial growth is left standing in its unaltered form in an atmosphere of normal levels of temperature and humidity. When the same silicon wafer as used in the conventional method was tested by following the procedure of Test Run 2, the resistivity exhibited by the sample after it had been left standing in an atmosphere having a relative humidity of not more than 50% was substantially constant without reference to the film thickness of the epitaxial layer as indicated by the curve 1c in Fig. 3. Example 2: The resistivity in an atmosphere having a relative humidity of 50% showed no discernible change with the passage of time and equalled the resistivity after the growth of the epitaxial layer and coincided substantially with the magnitude determined by the C-V method. In atmospheres having the degrees 60% and 95% of relative humidity, the resistivity showed changes with the passage of time, though more slowly than the change found in Example 1. The description given thus far clearly indicates that the method of determining the resistivity according to the invention allows the true resistivity of an n-type epitaxial layer to be accurately and infallibly determined by treating the surface of the n-type epitaxial layer for hydrogen termination and then measuring the sheet resistance of the n-type epitaxial layer by the four point probe method, with the surface kept fast in the state of hydrogen termination. This determination of the resistivity brings about the effect of facilitating guaranty of the quality of a silicon wafer and contributing to exaction of the management of the production process. The methods for determination of the resistivity according to the first and second embodiments of this invention allow the surface of a given n-type epitaxial layer of a sample to be treated for hydrogen termination easily and conveniently, enable the state of hydrogen termination to last stably, and permit the true resistivity of the sample to be determined accurately and infallibly. In the case of the method according to the second embodiment of this invention, even when the sample is not stored in an atmosphere having prescribed degrees of temperature and humidity, the accurate determination of the true resistivity is attained because this determination is carried out before the formation of a natural oxide film takes place. While there have been shown and described present preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practiced within the scope of the following claims.

Claims: A method of determining the resistivity of an n-type epitaxial layer formed on a silicon substrate, characterised by subjecting the surface of said n-type epitaxial layer to hydrogen termination and then measuring by means of the four point probe method the sheet resistance of said surface of said n-type epitaxial layer whilst still in the hydrogen terminated state and calculating the resistivity therefrom. A method according to Claim 1, wherein the measurement of the sheet resistance of said n-type epitaxial layer is accomplished by the steps of: growing said n-type epitaxial layer on a silicon substrate; displacing the inner gas of an epitaxial growth reactor with hydrogen gas and cooling said silicon substrate to a temperature below 400°C, thereby effecting said hydrogen termination of the surface of said n-type epitaxial layer; subsequently displacing said hydrogen gas in said epitaxial growth reactor with an inert gas and simultaneously cooling said silicon substrate to normal room temperaure; removing said cooled silicon substrate from said epitaxial growth reactor; and measuring sheet resistance of said n-type epitaxial layer by the four point probe method. A method according to Claim 1, wherein said n-type epitaxial layer is contacted with a dilute aqueous hydrofluoric acid solution to remove natural oxide film formed on the surface of said n-type epitaxial layer and, at the same time, effect said hydrogen termination, rinsed with deionized water, dried, and subjected to measurement of said sheet resistance by the four point probe method, said measurement being conducted in advance of any subsequent formation of an oxide film on the surface of said n-type epitaxial layer . A method according to any one of Claims 1, 2 and 3, wherein the sheet resistance measured by the four point probe method is multiplied by the film thickness of said n-type epitaxial layer, determined by the Fourier-Transform Infrared Spectroscopy method or the spreading resistance method, and the constant π/ln2 to obtain said resistivity.

Inventors: KATAYAMA MASATAKE, MITANI KIYOSHI, SAISU SHIGENORI, SUMIYA HIROSHI, KATAYAMA, MASATAKE, MITANI, KIYOSHI, SAISU, SHIGENORI, SUMIYA, HIROSHI
Applicants: SHINETSU HANDOTAI KK, SHIN-ETSU HANDOTAI COMPANY LIMITED
Application year: 1994
Main classification: H01L21
IPC codes: G01R27, G01N27, H01L21